- WELCOME TO ACIDS AND BASES PART 5.
THIS PART OF THE VIDEO TUTORIAL SERIES WILL FOCUS
ON THE CONCEPT OF KW,
AND ITS RELATIONSHIP TO HYDROGEN AND HYDROXIDE ION CONCENTRATION.
LET'S GET STARTED.
ALL RIGHTY. WATER, IS IT AN ACID OR A BASE?
HMM, LET'S LOOK AT THIS.
FROM OUR BRONSTED-LOWRY VIDEO TUTORIAL,
WE'VE LEARNED TO RECOGNIZE CONJUGATE ACID-BASE PAIRS.
SO, IF YOU'RE NOT FAMILIAR WITH THIS CONCEPT,
PLEASE GO BACK TO THE EARLIER VIDEO.
SO, TO FIGURE OUT THE ACIDS AND BASES,
WE LOOK AT THE REACTANTS, AND WE COMPARE,
AND WE SEE THAT THE HCL LOST TO PROTON,
SO THAT WOULD MAKE IT THE ACID. AND THEN THE CHLORIDE,
BECAUSE REACTANTS AND PRODUCTS DIFFER BY ONE PROTON,
WE WOULD DESCRIBE THE CHLORIDE AS THE CONJUGATE BASE.
SO, THIS WOULD BE OUR FIRST CONJUGATE ACID-BASE PAIR.
SO, IN EVERY ACID-BASE REACTION THERE'S GOING TO BE AN ACID
AND A BASE,
AND THE WATER IS SERVING IN THE ROLE OF THE BASE
BECAUSE IT ACCEPTED THE PROTON FROM THE ACID
TO CREATE THE HYDRONIUM ION.
SO, THAT WOULD BE OUR CONJUGATE ACID.
SO, HERE WE'VE LABELED THE CONJUGATE ACID-BASE PAIRS
OF THE FIRST REACTION.
SO, IN THIS REACTION, WATER WAS A BASE.
NOW, LET'S LOOK AT OUR NEXT REACTION.
WE LOOK AT OUR REACTANTS AND COMPARE THEM TO THE PRODUCTS.
AND HERE WE SEE THAT THE AMMONIA RECEIVED,
ACCEPTED A PROTON.
SO, IT TOOK THE ROLE OF THE BASE.
SO, THE AMMONIUM ION WOULD BE THE CONJUGATE ACID,
AND SO WE FOUND THE FIRST CONJUGATE ACID-BASE PAIR.
CONJUGATE ACID-BASE PAIRS ALWAYS OCCUR
ACROSS THE REACTION ARROWS.
WATER IN THIS CASE DONATED A PROTON TO THE AMMONIA.
SO, IN THIS EXAMPLE, WATER ACTED LIKE AN ACID AND SO THEN
THAT MEANS THE HYDROXIDE WOULD BE THE CONJUGATE BASE.
SO, THERE IS OUR SECOND CONJUGATE ACID-BASE PAIR.
IS WATER AN ACID OR A BASE? YES.
WHICH DOESN'T REALLY MAKE ANY SENSE, RIGHT?
IT'S AMPHOTERIC. WATER CAN ACT AS AN ACID OR A BASE.
SO, IT DEPENDS, EVERYTHING IS RELATIVE.
SO, IN THIS FIRST EXAMPLE, HYDROCHLORIC ACID
IS A STRONGER ACID THAN WATER, SO WATER TAKES THE ROLE
OF THE BASE.
IN THE SECOND EXAMPLE, AMMONIA IS A STRONGER BASE THAN WATER,
SO WATER TAKES THE ROLE OF THE ACID.
SO, WATER PLAYS OFF THE OTHER ELEMENTS IN THE REACTION.
SO, THAT LEADS US TO LOOK AT THE SELF-REACTION OF WATER.
SO, IN THEORY, WATER COULD REACT WITH ITSELF, RIGHT?
AND SO WE COULD SAY THIS MOLECULES ACTING AS THE ACID
AND THIS MOLECULE IS ACTING AS THE BASE,
AND SO THE BASE WOULD ACCEPT THE PROTON
AND THIS WOULD BE OUR CONJUGATE ACID.
AND THEN WE WOULD HAVE OUR HYDROXIDE
WHICH WOULD BE OUR CONJUGATE BASE.
SO, FROM HERE, WE CAN DEVELOP THE CONCEPTS OF A SOLUTION
BEING NEUTRAL, ACIDIC, OR BASIC.
SO, WHEN WE SAY A SOLUTION IS NEUTRAL, WE'RE GOING TO SAY
THAT THE HYDRONIUM AND THE HYDROXIDE CONCENTRATIONS
ARE EQUAL.
SO, THE CONCENTRATION OF HYDRONIUM
EQUALS THE CONCENTRATION OF HYDROXIDE.
IN AN ACIDIC SOLUTION, WE ARE GOING TO SAY
THAT THERE IS A HIGHER CONCENTRATION OF HYDRONIUM
THAN THE HYDROXIDE.
AND IN A BASIC SOLUTION WE WILL SAY THAT THE BASE
HAS A HIGHER CONCENTRATION THAN THE HYDRONIUM, RIGHT?
SO, YOU GOT TO KNOW YOUR GREATER AND LESS THAN, RIGHT?
SO, THIS IS GREATER. SO, AN ACIDIC SOLUTION
HAS A GREATER CONCENTRATION OF THE HYDRONIUM IONS
AND A BASIC SOLUTION HAS A GREATER CONCENTRATION
OF HYDROXIDE.
AND WE UNDERSTAND THAT IN WATER, WE WILL ALWAYS HAVE
A MIX OF BOTH.
SO, SOME PEOPLE THAT HAVE TRAVELLED THIS JOURNEY
BEFORE US, THEY DID A BUNCH OF MEASUREMENTS WITH WATER,
AND THEY MADE SOME IMPORTANT OBSERVATIONS THAT CAN HELP US
PERFORM CALCULATIONS.
WE HAVE THE KW EXPRESSION.
THE W STANDING FOR WATER.
SO, WHEN WE'RE IN AQUEOUS ENVIRONMENT,
WHAT WE'VE LEARNED IS THAT IF WE MULTIPLY THE HYDRONIUM
CONCENTRATION BY THE HYDROXIDE CONCENTRATION
THEY'RE ALWAYS GOING TO ADD OR MULTIPLY TO 10 TO THE MINUS 4.
SO, THESE CONCENTRATIONS, WE WILL POUR IN M FOR MOLARITY,
REMEMBERING THAT THAT'S MOLES PER LITER.
SO, THE UNITS, KIND OF, SLIP AWAY HERE
BUT IT'S IMPORTANT TO REALLY BE AWARE THAT IF YOU'RE DOING
THESE TYPE OF CALCULATIONS YOU ALWAYS DO THEM WITH MOLARITIES.
SO, HERE WE ARE--HERE'S THE INFORMATION FROM BEFORE,
BUT NOW IF WE TAKE OUR BASIC DEFINITION OF ACIDIC,
NEUTRAL, AND BASIC, AND LET'S CONNECT IT TO THE KW EXPRESSION.
AND I THINK SOMETIMES IT'S EASIER TO START AT NEUTRAL.
SO, IF THE HYDRONIUM AND THE HYDROXIDE CONCENTRATIONS
ARE NEUTRAL AND THEY HAVE TO MULTIPLY TOGETHER
TO MAKE 10 TO THE -14, THEN WHY DO WE KNOW?
THE HYDRONIUM ION CONCENTRATION IS GOING TO BE 10 TO THE -7.
AND BECAUSE OF ALL THESE COEFFICIENTS OF ONE,
DON'T BE SURPRISED IF I START IGNORING THEM.
WHEN YOU READ INFORMATION, DON'T BE SURPRISED.
SO, IF WE SEE AN EXPONENTIAL TERM WITHOUT A COEFFICIENT
WE ALWAYS ASSUME ONE.
SO, FOR ACIDIC, WE HAVE A GREATER HYDRONIUM CONCENTRATION,
SO MATHEMATICALLY WITH THESE NEGATIVE EXPONENTS, RIGHT,
RECOGNIZE THAT THAT MEANS THAT THE HYDRONIUM CONCENTRATION
IS GOING TO BE GREATER THAN 10 TO THE -7.
SO, SINCE THESE ARE NEGATIVE EXPONENTS, RIGHT?
THE EXPONENTIAL NUMBER BECOMES SMALLER.
AND THEN FOR BASE, WE WILL HAVE A BASIC ENVIRONMENT
HAS MORE HYDROXIDE.
SO, IF THERE'S MORE HYDROXIDE THAT MEANS WE'RE GOING TO HAVE
LESS HYDRONIUM.
SO, LET'S PRACTICE USING THE KW EXPRESSION
WITH SOME DIFFERENT SOLUTIONS.
SO, FOR EXAMPLE, COFFEE HAS A HYDRONIUM CONCENTRATION OF
1 x 10 TO THE -5 MOLAR.
LET'S CALCULATE THE HYDROXIDE CONCENTRATION,
AND LET'S DETERMINE WHETHER OR NOT THE COFFEE IS ACIDIC
OR BASIC.
WELL, SINCE THIS QUESTION HAS ALREADY GIVEN US
THE HYDRONIUM CONCENTRATION,
WE CAN ALREADY ANSWER ACIDIC OR BASIC, RIGHT?
WHAT'S THE RELATIONSHIP BETWEEN 10 TO THE -5.
AND 10 TO THE -7, RIGHT?
SO, THE 10 TO THE -5 THAT CAME FROM OUR QUESTION.
WHICH NUMBER IS BIGGER?
10 TO THE -5 IS A BIGGER NUMBER.
SO, SINCE THIS CONCENTRATION IS GREATER THAN
10 TO THE -7,
COFFEE IS ACIDIC, WHICH IS WHY FOR SOME OF US,
IT CAN UPSET OUR TUMMIEs.
NOW, LET'S GO AHEAD AND CALCULATE
THE HYDROXIDE EQUATION.
SO, BASICALLY THIS IS A VERY SIMPLE ALGEBRAIC EXPRESSION.
THE KW EQUALS 10 TO THE -14 AND WE KNOW THAT EQUALS
THE HYDRONIUM x THE HYDROXIDE.
AND WE KNOW THE HYDRONIUM CONCENTRATION,
SO WE CAN PLUG THAT IN
10 TO THE -14 = 10 TO THE -5 TIMES THE HYDROXIDE.
SO, WE JUST USE SOME SIMPLE ALGEBRA DIVIDE BOTH SIDES
BY THE 10 TO THE -5, RIGHT?
AND OUR HYDROXIDE CONCENTRATION WILL BE 10 TO THE -9 MOLAR
WE'LL PUT THE ONE BACK IN, BUT IT'S UNDERSTOOD.
OKAY. WE'LL TRY TWO MORE,
AND THAT'LL WRAP UP THIS TUTORIAL.
SO, THE DIGESTIVE SOLUTION IN OUR STOMACHS
HAS A HYDROXIDE CONCENTRATION OF 10 TO THE -12.
WHAT IS THE HYDRONIUM ION CONCENTRATION OF THIS SOLUTION?
SO, I'M GOING TO WAIT ON THE ACIDIC OR BASIC PART
UNTIL WE'VE DONE OUR CALCULATION, RIGHT?
SO, IT'S GOING TO BE EXACTLY THE SAME,
THE KW ALWAYS EQUALS 10 TO THE -14,
AND IT ALWAYS EQUALS THE HYDRONIUM TIMES THE HYDROXIDE.
HOWEVER, THIS TIME, WE'VE BEEN GIVEN THE HYDROXIDE.
SO, OUR HYDRONIUM CONCENTRATION WILL BE
10 TO THE -14 DIVIDED BY 10 TO THE -12.
SO, THAT WOULD GIVE US 10 TO THE -2 MOLAR.
AND SO IF WE LOOK AT 10 TO... SO, THAT'S THE ANSWER
TO THE FIRST PART.
AND THEN WE LOOKED AT THE RELATIONSHIP
BETWEEN 10 TO THE -2 AND 10 TO THE -7.
10 TO THE -2 IS A LARGER NUMBER.
AND SO, WHEN WE HAVE A HIGH HYDRONIUM ION CONCENTRATION,
WE WOULD SAY AND OF COURSE THE DIGESTIVE SOLUTION
IN OUR STOMACHS IS STOMACH ACID.
SO, THIS MAKES SENSE, RIGHT?
OKAY. AND NOW, LET'S WORK ONE LAST ONE, LIQUID SOAP.
LIQUID SOAP HAS A HYDROGEN ION CONCENTRATION
OF 1 TIMES 10 TO THE -9 MOLAR.
WHAT IS THE HYDROXIDE CONCENTRATION OF LIQUID SOAP?
SO, BY NOW THIS SHOULD BE GETTING PRETTY COMFORTABLE
FOR YOU,
AND WE WOULD HAVE THE HYDRONIUM AND THE HYDROXIDE.
SO, IN THIS EXAMPLE, WE WERE GIVEN THE HYDRONIUM,
SO WE WOULD PLUG THAT IN.
AND THE HYDROXIDE SOLUTION WOULD BE 10 TO THE -14
DIVIDED BY 10 TO THE -9
WHICH WOULD BE TEN TO THE MINUS FIVE MOLAR HYDROXIDE.
IS SOAP ACIDIC OR BASIC?
WE'LL, HERE'S OUR HYDRONIUM ION CONCENTRATION
10 TO THE -9 VERSUS 10 TO THE -7.
SO, IN THIS CASE, RIGHT, THE 10 TO THE -9 IS SMALLER.
SO, SINCE THE HYDRONIUM ION CONCENTRATION IS LESS THAN
10 TO THE -7,
WE RECOGNIZE THAT SOAP WOULD BE A BASIC SOLUTION.
SO, THIS CONCLUDES OUR TUTORIAL ON KW.
PLEASE TAKE SOME TIME NOW TO WORK A FEW HOMEWORK PROBLEMS
TO REINFORCE YOUR UNDERSTANDING.